One of the effects provided in conventional electronic organs is the ability to generate a repeat feature in which a single note, or two notes at some predetermined interval, such as an octave, are played repetitively by depressing a single key. The repeat pattern continues as long as the key is depressed. In connection with the repeat function, it is desirable to provide the musician the option of having a short or a long release time for the repetitive function. With a long release time or "sustain" it is necessary to force the repeat function to continue after the key is released while the overall amplitude of the individual notes are gradually reduced. In U.S. Pat. No. 3,910,150 there is described an arrangement for providing an octave repeat with a sustained interval subsequent to key release in a computer organ. The computer organ described in the patent has tone generators which compute in real time the amplitude of successive sample points of a musical wave shape.
The present invention is directed to an arrangement for providing a sustained repeat function in a digital organ of the type described in copending application Ser. No. 603,776, filed Aug. 11, 1975, entitled "Polyphonic Tone Systhesizer", now issued as U.S. Pat. No. 4,085,644.This application describes a digital computer organ having a plurality of tone generators, each tone generator including a computation mode in which data defining the relative amplitudes of a plurality of sample points of the desired musical wave shape are calculated. This data is then converted to an analog voltage having a fundamental frequency corresponding to the pitch of the key to which the note generator is assigned. A circuit for detecting operation of the individual keys and assigning note generators to those keys is described in copending application Ser. No. 619,615, filed Oct. 6, 1975, entitled "Keyboard Switch Detect and Assigner" issued as U.S. Pat. No. 4,022,098. The waveshape of a particular tone generated by a tone generator is in turn controlled by an envelope generator of the type described in copending application Ser. No. 652,217, filed Jan. 26, 1976, entitled "ADSR Envelope Generator", now issued as U.S. Pat. No. 4,079,650.
The present invention is directed to an improvement in the tone synthesizer of the type described in these copending applications, and particularly provides a modification to the ADSR amplitude generator by which the overall amplitude of successive notes produced by one or more tone generators are gradually reduced down to zero over a controlled period of time.
This is accomplished, in brief, by providing a register for storing amplitude information for each tone generator. When a tone generator is assigned to a key, an initial amplitude valve is stored in the register. If the key has initiated a Repeat function in which the note is repeatedly played or played alternately with a second note, the circuit senses when the key is released and decrements the amplitude value stored in the resigter for that note each time the note is repeated following the release. When the amplitude value is reduced to 0, the tone generator is released for assignment to another key. The amplitude information from the register is applied to a digital-to-analog converter which generates a corresponding voltage which steps down in increments as the digital value is decremented. This step function voltage is used to control the overall amplitude of the audio signal and hence the loudness of the note on each successive playing of the note after the key is released.